That's not what undecidable means. Consider the halting problem, which is undecidable. It's trivial to produce Turing Machines which provably do or do not halt. What's impossible is proving that for every Turing machine. Some machines have a provable halting, but some do not. Similarly, if complexity class equality were undecidable it's not that you couldn't pick two complexity classes which are provably equal or disjoint, it's that there's a pair of complexity classes which exhibits that property. You don't know which pair it is, so you can't point to it specifically, you just know such a pair exists.

So you're partially correct; if P vs NP were proved undecidable, we would be able to decide it. But this doesn't mean P vs NP isn't undecidable, it just means you can't prove it's undecidable. It may be true, and there's nothing stopping you from believing it is, but it's not provable.